1. Field of the Invention
The present invention relates to a thin film of oxide superconductor possessing locally different crystal orientations and more particularly to a process for producing a thin film of oxide superconductor having a-axis (or b-axis) oriented thin film portions and c-axis oriented thin film portions on a common substrate and processes for preparing the same.
2. Description of the Related Arts
Oxide superconductors are expected to be used in a variety applications due to their higher critical temperatures than conventional metal superconductors. In fact, Y-Ba-Cu-O oxide superconductors possess a critical temperature above 80 K and Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O oxide superconductors possess the critical temperature above 100 K. However, the oxide superconductors possess crystal anisotropy in their superconducting properties. In fact, the highest critical current density is observed in a direction which is perpendicular to the c-axis of their crystal. From this fact, the direction of crystal orientation must be considered in actual utilization of oxide superconductors.
In the oxide superconductors to which the present invention is applicable, a-axis, b-axis and c-axis of their crystals are determined and known. It is also known that electric current flows in a plane defined by the a-axis and the b-axis. In the specification, only a-axis oriented thin film is referred to because the difference between an a-axis oriented thin film and a b-axis oriented thin film is not important from the view point of superconducting current flow. Orientation of these axes is usually defined by the direction of crystal axis with respect to a surface of the substrate. For example, "c-axis oriented thin film" means a thin film whose c-axis is perpendicular to the surface of substrate.
When the oxide superconductors are used in superconductor electronics for producing superconducting devices or integrated superconducting circuits, it is indispensable to prepare a thin flint of the oxide superconductors. However, the problem of crystal anisotropy becomes critical in the thin films of oxide superconductors.
In addition to this, in order to realize high-performance superconducting devices or integrated superconducting circuits of high density, it is requested to prepare, on a common surface of substrate, two kinds of superconducting wiring lines: one in which electric current flows in parallel with a surface of substrate and another in which electric current flows perpendicular to the surface of substrate. For example, in superconducting electrodes, current flows usually in parallel with the surface of substrate, while in interlayer superconducting wiring lines connecting different layers stratified on the substrate, current must flow perpendicular to the surface of substrate. Therefore, when oxide superconductor is used in high-performance superconducting devices or integrated superconducting circuits of high density, it is requested to deposit, on a common surface of a substrate, both of a c-axis orientated thin film of oxide superconductor in which the critical current density along the direction which is in parallel with the surface of substrate is higher than the other directions and an a-axis (or b-axis) orientated thin film of oxide superconductor in which the critical current density along the direction which is perpendicular to the surface of substrate is higher than the direction of c-axis.
Crystal orientation of the thin film of oxide superconductor can be controlled by selecting a film-forming temperature which is determined usually by substrate temperature. In fact, the a-axis oriented thin film can be realized at a substrate temperature which is lower by about 50.degree. to 100.degree. C. than a substrate temperature which is preferable to prepare the c-axis oriented thin film. From this fact, in a known process for preparing a thin film having a-axis oriented thin film portions and c-axis oriented thin film portions in a common substrate, one of the thin film portions is deposited firstly on a surface of the common substrate, then selected areas of the firstly deposited thin film portions are removed by an etching technique to produce patterned first thin film portions and after that, another thin film portions are deposited on the selected area.
This known technique, however, is not effective in actual production because it is necessary to effect two deposition steps of two different thin films under different conditions. Still more, the dimension of superconducting wiring lines obtained is limited due to tolerance or precision of the etching operation, so that a cross section of a superconducting wiring line obtained becomes unnecessarily large, resulting in that it is difficult to increase the density of superconducting circuits. There is also such danger that the thin film of oxide superconductor obtained is contaminated at etched margins or nonsuperconductor deposit on the etched margins. Still more, when etching operation is not effected satisfactorily, for example, when a firstly deposited c-axis oriented thin film is not removed completely but is left, the next thin film deposited on the firstly deposited c-axis oriented thin film will not become an a-axis oriented thin film but becomes a c-axis oriented thin film. Even if the firstly deposited thin film can be removed completely, the resulting surface is often roughened due to delicate operational conditions of etching. In such case also, it is difficult to realize a well-ordered a-axis oriented thin film of high quality.
The known process has another problems. Namely, when a c-axis oriented thin film is deposited firstly, and then an a-axis oriented thin film is deposited secondly, even if the etching operation is effected ideally, oxygen atoms escape out of the margin of the c-axis oriented thin film which are exposed by the etching operation, so that the stoichiometry of the thin film is spoiled at the margins of the c-axis oriented thin film, resulting in deterioration of superconducting properties at the margins. In this case, after deposition of an a-axis oriented thin film is effected, it is impossible to re-supply oxygen atoms to the marginal area of the c-axis oriented thin film by conventional oxygen supply techniques such as heat-treatment, because the marginal area is filled or closed with the a-axis oriented thin film and hence oxygen can't enter the c-axis oriented thin film due to such a fact that oxygen can enter only through the plane defined by a-axis and b-axis. Therefore, the superconducting device obtained sometimes does not work.
Still more, in the known process, it is very difficult to realize a perfect or satisfactory interface between a c-axis oriented thin film deposited firstly and an a-axis oriented thin film deposited secondly, resulting in that the capacity of electric current at the interface is limited. And, a weak junction caused by the contamination at the etching operation is often produced at the interface between the a-axis oriented thin film and the c-axis oriented thin film, resulting in that the obtained superconducting device shows non-linear property due to a tunnel current.
An object of the present invention is to solve the problems of the known process and to provide an improved thin film of oxide superconductor possessing locally different crystal orientations and two processes for preparing the thin film.